Electrodeposition coating is a method of coating in which an electric current is applied to an electrically-conductive object dipped for coating in an electrodeposition coating material tank, to cause solid matter contained in an electrodeposition coating material to be deposited on the surface of the object to be coated, whereby a coat is formed. Thus, the solid matter contained in the electrodeposition coating material is taken away by the object to be coated and is apt to gradually lessen; therefore, in order to obtain the desired coating thickness, coating performance, etc., it is necessary to maintain the proportion of the solid matter contained in the electrodeposition coating material at a given value.
The following heating residues method has hitherto been used to measure solid matter contained in an electrodeposition coating material.
(1) The weight (A grams) of a weighing pan is measured. PA1 (2) A sample of coating material is taken and put in the weighing pan, and the weight (B grams) of the weighing pan contains said sample is measured. PA1 (3) The weighing pan containing the coating material is put in a dryer and heated at a temperature of, for example, 105.degree. C. for 3 hours. After water, solvent and the like are evaporated, said weighing pan is gradually cooled in a desiccator. The weighing pan is taken out when cooled down to its normal temperature, and the weight (C grams) thereof is measured. PA1 (4) The proportion of solid matter to the coating material is calculated by means of the formula (C-A)/(B-A). PA1 (1) The weight ("a" grams) of a crucible is measured. PA1 (2) A sample of coating material is taken and put in the crucible, and the weight ("b" gram) of the crucible contained said sample is measured. PA1 (3) The crucible containing the coating material is put in a dryer and dried at a temperature of, for example, 150.degree. C. for 60 minutes so that water is evaporated. Then, said crucible is intensely heated (for about 30 to 60 minutes) by a gas burner so that organic matter is completely burned. After burning, the crucible is cooled in a desiccator, and the weight ("c" grams) thereof is measured. PA1 (4) The proportion of pigment in the coating material is calculated by means of the formula (c-a)/(b-a). PA1 measuring the attenuation L of an ultrasonic wave through the electrodeposition coating material and generating a signal regarding the attenuation of the ultrasonic wave; PA1 measuring the density .rho. of the electrodeposition coating material and generating a signal regarding the density; PA1 measuring the temperature T of the electrodeposition coating material and generating a signal regarding the temperature; PA1 calculating the proportion N of solid matter and the proportion W of pigment contained in the electrodeposition coating material, on the basis of the signals regarding said ultrasonic-wave attenuation, density and temperature; PA1 comparing the calculated proportions N and W of the solid matter and pigment contained in the electrodeposition coating material with reference values and generating control signals; and PA1 feeding supplementary coating material in accordance with said control signals. PA1 For the purpose of resolving the aforementioned problem, the present invention also provides an apparatus for controlling the composition of an electrodeposition coating material in an electrodeposition coating material tank, which comprises: PA1 an electrodeposition coating material tank for subjecting objects to be coated to electrodeposition coating; PA1 an ultrasonic-wave attenuation measuring part which measures the attenuation L of an ultrasonic wave through the electrodeposition coating material and which generates a signal regarding the attenuation of the ultrasonic wave; PA1 a density measuring part which measures the density .rho. of the electrodeposition coating material and which generates a signal regarding the density; PA1 a temperature measuring part which measures the temperature T of the electrodeposition coating material and which generates a signal regarding the temperature; PA1 an output circuit which compares the calculated proportions N and W of the solid matter and pigment contained in the electrodeposition coating material with reference values and which generates control signals; and PA1 a feed part for feeding supplementary coating material to said electrodeposition coating material tank, in accordance with said control signals. PA1 For the purpose of resolving the aforementioned problem, the present invention further provides a method for analyzing the composition of an electrodeposition coating material in an electrodeposition coating material tank, which comprises the steps of: PA1 measuring the attenuation L of an ultrasonic wave through the electrodeposition coating material and generating a signal regarding the attenuation of the ultrasonic wave; PA1 measuring the density .rho. of the electrodeposition coating material and generating a signal regarding the density; PA1 measuring the temperature T of the electrodeposition coating material and generating a signal regarding the temperature; and PA1 calculating the proportion N of solid matter and the proportion W of pigment contained in the electrodeposition coating material, on the basis of the signals regarding said ultrasonic-wave attenuation, density and temperature. PA1 For the purpose of resolving the aforementioned problem, the present invention further provides an apparatus for analyzing the composition of an electrodeposition coating material in an electrodeposition coating material tank, which comprises: PA1 an electrodeposition coating material tank for subjecting objects to be coated to electrodeposition coating; PA1 an ultrasonic-wave attenuation measuring part which measures the attenuation L of an ultrasonic wave through the electrodeposition coating material and which generates a signal regarding the attenuation of the ultrasonic wave; PA1 a density measuring part which measures the density .rho. of the electrodeposition coating material and which generates a signal regarding the density; PA1 a temperature measuring part which measures the temperature T of the electrodeposition coating material and which generates a signal regarding the temperature; and PA1 an arithmetic operation circuit which calculates the proportion N of solid mater and the proportion W of pigment contained in the electrodeposition coating material, on the basis of the signals regarding said ultrasonic-wave attenuation, density and temperature.
Solid matter contained in an electrodeposition coating material consists of resin and pigment. As for the proportion of the pigment to the solid matter, the proportion of pigment to be taken away so as to be deposited on an object to be coated is different from the proportion of pigment contained in the electrodeposition coating material; therefore, the proportion of the pigment in the coating material is apt to gradually change during electrodeposition coating. And if the proportion of pigment contained in a coat is too high, the smoothness of the coat is lowered and the coat becomes brittle. On the other hand, if said proportion is too low, problems such as poor corrosion resistance (the coat becomes liable to rust), change in the color of the coat, etc., arise. Accordingly, in order to obtain the desired coating finish and coating performance, it is necessary to maintain the proportion of the pigment in the electrodeposition coating material at a given value.
The following ashing method has hitherto been used to measure the proportion of pigment contained in an electrodeposition coating material.
However, a method of measuring solid content by using said heating residue method, and a method of measuring pigment contained in a coating material by using said ashing method, have the disadvantage that they require a great deal of expense, time and labor, since they comprise the steps of sampling, weighing, heating, gradual cooling, calculating, etc. Moreover, in those methods, no real-time measurement scan be made; therefore, they have the disadvantage that the proportions of solid matter and pigment are liable to change, etc.
As to methods for measuring the proportions of solid matter and pigment, Japanese Laid-Open Patent Publication Nos. 96296/88 and 26329/89 disclose a method of calculating the concentration of each of them on the basis of the attenuation of an ultrasonic wave through a coating material. In this method, real-time measurement is possible, and a high precision of measurement is obtained. But, at the time of measuring solid content, it is necessary that a change in ultrasonic-wave attenuation due to a change in the proportion of pigment be so small that it can be ignored. Also, at the time of measuring the proportion of pigment, it is necessary that the influence of a change in ultrasonic-wave attenuation due to a change in the proportion of solid content be minimal. Thus, this method has the disadvantage that the reliability of measured values is lowered in the case where the proportions of solid content and pigment content are simultaneously changed to a great extent. Furthermore, Japanese Laid-Open Patent Publication No. 70737/80 discloses a method of measuring the proportions of solid content on the basis of the attenuation of an ultrasonic wave through a suspension. However, this method also has the disadvantage that in the case where the proportions of solid content and pigment content are simultaneously changed to a great extent, the rate of change for each of them cannot be detected.
Thus, the problem which the present invention seeks to resolve is that it is not possible to make a real-time measurement of the proportions of solid matter and pigment contained in an electrodeposition coating material.